The present invention relates generally to fiber optic communications where an electrical signal is converted to a light signal, which is transmitted through a fiber to a distant receiver, where it is converted back into the original electrical signal. Such communications systems have many advantages. A signal can be sent to relatively long distances without being amplified; there are no interference problems from nearby electrical fields; a relatively large number of electrical signals can be concurrently transmitted; and the fiber is relatively light and small.
One of the problems with fiber optic communications systems is that the light signal fades or loses power, in other words, becomes attenuated, as it travels along the fiber. Light is attenuated by being absorbed into the fiber, by leaking out of the fiber (due to imperfections or due to excessive bending of the fiber), by being scattered due to Rayleigh scattering, and by being reflected due to Fresnel reflection (which occurs when there is a sudden change in the density of the material through which the light is traveling, such as occurs at the ends of the fibers, and at fiber breaks).
It is important to know how much attenuation occurs in a length of fiber before the fiber is used in a communications system, and also, it is important to determine whether excessive power loss occurs once the fiber has been placed in a communications system. Such excessive power loss may be due to excessive bending of the fiber, due to fiber damage caused by excavators, hammers, and the like, and due to imperfect coupling or splicing of fiber ends.
Once the fiber is used in a communications system, it is important to assess the magnitude of any attenuation through the entire length of the fiber, and also to detect where any excessive power loss is occurring so that remedial action may be taken. Also, many job specifications require that in order for a contractor to be paid for placing the fiber optic communications system, the power loss at any splice must not exceed a certain magnitude.
A widely used method of determining light attenuation in a fiber utilizes an optical time domain reflectometer (xe2x80x9cOTDRxe2x80x9d). In general, an OTDR sends one or more pulses of laser light through the optic fiber. Each pulse has a predetermined width or time duration, and the interval between pulses is also predetermined. The pulse of laser light traveling through the fiber is somewhat akin to a flashlight being shined into fog (which creates a backscatter of light) or shined through a window (which causes a reflection of some light). The OTDR measures the amount of light being sent backward through the fiber as being representative of the amount of light attenuated. Although the OTDR measures only the amount of light being sent back through the fiber, and not the amount of light being transmitted through the fiber, there is a very close correlation between the two amounts.
The OTDR includes a very precise photodetector that measures the power level of light coming back through the fiber. The OTDR also includes a very precise and sensitive clock that knows when the laser pulse is fired into the optic fiber and when light is sensed by the photodetector. Since light travels in a vacuum faster than it travels in matter (the ratio between the two being called the index of refraction of the matter) and since the index refraction of the fiber is generally known, the OTDR may calculate the distance along the length of fiber where light has been attenuated and the magnitude of that attenuation.
The OTDR may be coupled with a controller to create a graph of light signal level (on the Y axis) and distance along the optic fiber (on the X axis) and to plot a series of data points based upon a sampling of the photodetector and the clock. The series of points may be connected together in what is known as a trace.
The accuracy of the signal level trace is dependent upon the accuracy of the photodetector as well as the correlation between the amount of light traveling back through the fiber as compared with the amount of light transmitted through the fiber. The accuracy of the distance of the feature in the fiber causing the signal loss from the end of the fiber into which the laser is fired is dependent upon the pulse width, the precision of the clock, and the accuracy of the index of refraction (throughout the length of the fiber) and to some degree is dependent upon the spacing of the data points that are used to form the trace.
Normally, the wavelength of the laser light in the OTDR is the same as the wavelength of light to be transmitted for communications purposes through the fiber. Also, the fiber is normally tested by sending a laser pulse down each end of the fiber in what is known as a bi-directional test.
FIG. 1 shows an exemplary trace utilizing an OTDR. The relatively gently, linearly sloped regions indicate attenuation due to backscatter, whereas the more pronounced sloped regions of the trace indicate so-called xe2x80x9ceventsxe2x80x9d which cause more severe power loss, as best shown in FIG. 2. Note that events caused by certain phenomena (e.g., a splice) possess a characteristic, unique signature or waveform indicative of the type of phenomena. These events are of special importance because they suggest some irregularity in the fiber that might need correction or remedy. For example, a poor splice of fiber ends (usually through either melting the ends together in a fusion or through a mechanical connector) might require that the splice be remedied. Also, cracking of the fiber that might have been caused by an excavator inadvertently pressing against the fiber, might require corrective action. Consequently, it is important to locate where events happen within the fiber and to determine the magnitude of power loss at each event so that the problem may be remedied efficiently. For example, the location might be at a manhole, at a pedestal, on a particular pole, or at a construction site.
Although an event happens at a particular point or within an extremely short range of distance within a fiber (such as where a fiber end is spliced to the end of a different fiber), the trace will show that the power loss occurs over a short distance as best shown in FIGS. 1-3. For example, the OTDR trace of a fiber might indicate that the event of a splice starts at 10.0 kilometers and ends at 10.1 kilometers from the fiber end through which the laser is fired, where in actuality, the splice is 10.005 kilometers from such end. Thus, an event on a trace is said to have xe2x80x9cextentxe2x80x9d. Such extent is caused by the width of the laser pulse as well as the natural intervals caused by data point sampling of the return light. Each event is also deemed to have a xe2x80x9cstartxe2x80x9d and an xe2x80x9cendxe2x80x9d, in accordance with conventional standards, for example, as indicated by the two vertical dashed lines in FIG. 3. Also, not every relatively sudden attenuation is deemed to be an xe2x80x9ceventxe2x80x9d. Various conventional parameters determine whether a power loss has characteristics sufficient to deem the power loss an xe2x80x9ceventxe2x80x9d.
Most fiber optic cables include a plurality of optic fibers, with cable being currently commercially available with up to 432 such fibers in what is known as xe2x80x9c432 countxe2x80x9d cable. Each fiber within the cable is coded and is typically tested for light attenuation using an OTDR in the manner described above.
In addition to the imperfections of distance accuracy previously mentioned, the problem of locating an event is further compounded because a fiber optic cable may possess many strands of fiber helically wrapped around a central supporting core such that the length of the outer fibers is longer than the length of the inner fibers over the distance of the cable. When considering the different lengths of fiber wrapped in a cable, the same event may be located at different distances from the fiber ends. For example, if there is a splice of fibers at a particular manhole, the OTDR trace might show that the event happens at a location starting at 10.0 kilometers and ending at 10.1 kilometers in fiber A, but the event starts at 10.007 and ends at 10.111 in fiber B, which is wrapped further outwardly in the same cable. From simply reading the trace of each fiber, it is difficult to determine whether the same event is causing the power loss at a different distance in each fiber. A conventional method of adjusting the distance of an event along a fiber is to determine the distance of the last or so-called xe2x80x9cendxe2x80x9d event for each fiber, which by assumption is the termination or end of the fiber, as shown in FIG. 2. Since each of the fibers ends at the same location along the length of the cable, differences in the distance of the end event in each fiber grouped in a cable are representative of the differences in the lengths of each fiber. An assumption can further be made that the percentage of difference in the lengths of the fibers can be extrapolated along the entire length of the cable to adjust relative distances of events occurring in each fiber. For example, if the end event in fiber A is deemed to start at 50.00 kilometers, and the end event fiber B is deemed to start at 51.00 kilometers, then the distances of all of the events in fiber B will be shortened by 2% relative to the events in fiber A. Such a distance adjustment is made for each of the fibers in a cable. It is noteworthy that some optic fibers are placed in parallel (without any helical wrapping) in so-called ribbon cable. Although the same adjustment method may be utilized with ribbon cable, typically no significant adjustment will need to be made.
For certain events, such as a splice, power loss is conventionally calculated in either of two methods. As shown in FIG. 3, a so-called xe2x80x9c2 point attenuation correctionxe2x80x9d method simply calculates the difference of the signal level between the backscatter at the start of the event and at the end of the event and subtract normal fiber attenuation. The so-called xe2x80x9cLSAxe2x80x9d or least squares analysis calculates a linear slope of the signal level due to backscatter both before (i.e., upstream) and after (i.e., downstream) of the event and extrapolates each slope to the start of the event. The slope is determined at a selected range of distance before and after the event using so-called xe2x80x9ccursorsxe2x80x9d. The difference between the extrapolated signal level prior to the start of the event and the extrapolated signal level after the event is another measure of power loss caused by the event, as also shown in FIG. 3.
Typically, the LSA method calculates a more accurate amount of power loss caused by an event, and typically using more data points results in a better estimate. However, where two events are close together, it will be impossible to accurately use an LSA cursor, and in such situations the two point attenuation corrected method is used.
Manual techniques have been used to create a template that designates a start point for the event, and for placing LSA cursors before and after the event, and where the cursors cannot be used, selecting the start and end points for the event so that the two-point attenuation corrected method may be used. In the example shown in FIG. 5, there are traces for five fibers, A, B, C, D, and E. These five fibers might be part of a 432 count cable. In reviewing the five traces, a technician might conclude that there is a splice at a distance generally designated as region 1, since each of the traces shows a splice type event generally in that region. The technician might or might not conclude that there is a splice at the distance generally designated as region 2, since two out of the five fibers show a splice type event generally at that distance. The technician might review traces of additional fibers in an attempt to better evaluate whether a relatively high percentage of traces show a splice type event occurring in region 2. Obviously, it would be extremely laborious to review traces of all 432 fibers. Likewise, a technician might or might not believe that a splice occurs at a distance generally designated as region 3 in FIG. 5, and might further review traces of other fibers to make an evaluation. The technician would probably conclude that there is no splice at a distance generally designated by region 4 in FIG. 5, since none of the five traces shows an event occurring. Such a conclusion might be erroneous in that even though the five exemplary or sample traces do not show an event in region 4, it is possible that perhaps seventy-five traces of other fibers do show an event in region 4. It should be appreciated that there is some estimation and guess-work as to whether the sample or exemplary traces indicate that a splice exists within a region along an optic fiber cable. It would be very laborious to evaluate all fibers, and also there is usually no uniform standard as to how many or what percent of traces must show an event within a particular region in order to conclude that the region includes a splice.
A further problem in existing methodology is that once a region is deemed to include a splice and the calculation must be made as to the power loss experienced by each fiber in the region of the splice, there is some uncertainty as to where the start point should be designated, where the LSA cursors should be placed (in an LSA method) and where the start and end points should be designated (in a two-point attenuation corrected method). Consequently, the start point is often selected by simply xe2x80x9ceyeballingxe2x80x9d the sampling of the events in a region and, by using experience and discretion, selecting appropriate LSA cursor locations and end points. Such an xe2x80x9ceyeballxe2x80x9d methodology injects non-uniformity in connection with power loss calculations, not only from technician to technician, but also by the same technician. It should also be appreciated that the training of technicians to interpret the traces and to select a template is extensive and that the existing process is laborious. Often a technician spends two to four hours preparing a template over a single cable. The process is also fraught with potential for innocent and negligent errors, as well as reckless and intentional mistakes.
The present invention provides a substantially uniform methodology for locating splices in optic fiber cable and also for setting up a splice template for calculating the power loss at splices in optic fibers in a cable, in an automated manner.
The present invention relates to a method of determining the location of splices and of calculating event start and end locations, the type of loss estimate, and LSA cursors locations and lengths, which will be used to calculate the power loss at splices in optic fibers in a cable.